Two-way shape memory effect of NiTi alloy induced by constraint aging treatment at room temperature
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I. INTRODUCTION
THE near-equiatomic NiTi shape-memory alloy (SMA) exhibits a shape-memory effect (SME) that is related to the reversible martensitic transformation. The SME in NiTi SMAs refers to the recovery of deformations of up to 8 pct upon heating above the austenite finishing temperature (Af). Generally, two shape-memory phenomena can be distinguished—a one-way SME and a two-way shape-memory effect (TWSME). The basic difference between the one-way SME and the TWSME is that no reverse change of a shape occurs in the case of the one-way SME after subsequent cooling, whereas the TWSME is characterized by a shape change during cooling. The TWSME exhibits switching between a “cold” shape and “hot” shape upon heating and cooling, respectively. A special case of the TWSME is called the all-round SME.[1,2] The all-round SME occurs in Ni-rich NiTi alloys and is strongly affected by the Ni concentration in a specimen. Basically, the all-round SME in NiTi alloys is associated with the internal-stress fields between coherent precipitates and the NiTi matrix. The Ni-rich precipitates are formed during a constrained aging process and have a special orientation arrangement by the constrained stress. Most often, the all-round SME is observed in Ni-rich NiTi alloys with the Ni14Ti11 precipitates dispersed in the matrix.[1,2,3] In the literature, training procedures consisting of several to many thermomechanical cycles preceded by an optimal heat treatment are required to develop the TWSME. During training processes, a directed-stress field is introduced in the material. The resulting residual-stress field assists the formation and growth of preferentially oriented martensite variants when the martensitic transformation proceeds in a specimen on cooling.[4–8] In the past, several different kinds of training procedures have been suggested: (1) a repeated one-way SME process, in which better results are received if the specimen is intentionally constrained during heating;[9]
CHING-YIG CHANG, Engineer, is with United Microelectronics Co., Hsinchu 30043, Taiwan, Republic of China. DAVID VOKOUN, Postdoctor, and CHEN-TI HU, Professor, are with the Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30043, Taiwan, Republic of China. Manuscript submitted May 15, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
(2) a repeated pseudoelastic process;[10] and (3) repeated cycles of load-free heating followed by cooling under weak load.[11] It was recognized that the effectiveness of training procedures also depends on the applied heat treatment.[11] The disadvantage of the aforementioned methods is that a relatively complicated training device has to be applied to perform any of them, in addition to the great amount of time and labor required. The aim of the present work is to develop a new training procedure to induce the TWSME in a Ni-rich NiTi SMA by employing single training cycles, as constraint aging at room temperature preceded by a proper heat treatment (preaging). A similar procedure was stu
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